What you are describing, Mike, is free oxygen, that is molecules of entrained oxygen(air!) that is not part of the H2O combination. In the case of an automobile cooling system, it's a closed system, opened only when you check the coolant level or boil some of it into the overflow vessel(15psig). It can also happen if the water pump cavitates - but that usually wont happen unless the water level has gotten too low Boiling has happened only once to me - two years ago and that was because I didn't have the fan wired correctly and was fixed immediately after cool down. As far as my overflow/catch can - it's pretty high plus a good size compared to many. I did move it from one side to the other recently, but it's at about the same height.
Another plus with using antifreeze is that it does contain anti corrosives, water wetting agents, raises the boiling point a few degrees.
This is my coolant recovery system:
http://i70.photobucket.com/albums/i8...d/DSCN1748.jpg